1. Field of the Invention
The present invention relates in general to a vehicle braking system, and, more specifically, to a power management system for a vehicle braking system having electromagnetic and friction braking.
2. Description of the Related Art
Hybrid braking systems for vehicles are well known. These braking systems usually include electromagnetic and friction braking capabilities. The use of electromagnetic braking improves efficiency in vehicles by recovering kinetic energy produced by the vehicle with an electric machine wherein the electric machine generates electric energy that is applied to electromagnetic retarders (also referred to as eddy current devices) within the braking system. In some instances, the electric machine is unable to generate a desired amount of braking energy in which to stop the vehicle. When this occurs, prior art braking systems have disclosed methods of compensating for the inadequacy of the electromagnetic brakes by utilizing the friction braking capabilities. However, these prior art systems have proved to be largely inefficient and the system transitions from electromagnetic braking to friction braking and vice versa are noticeable to the vehicle operator.
Prior art systems have been subject to low braking forces during the initial phase of braking from the eddy current devices. The present inventors have discovered that this deficiency results from the electric machines being inadequately powered up during that time. For example, when the vehicle operator first depresses the brake pedal and excitation current energizes the electric machine, the speed dependent characteristics of the electric machine prevents the generation of an adequate amount of power for the braking system which renders the electromagnetic braking functions inadequate.
It would therefore be desirable to provide a hybrid braking system that efficiently distributes the energy generated within the braking system, utilizes a supplemental power source for the eddy current devices when the electric machine is unable to generate an adequate amount of power, and provide a seamless braking feel to the vehicle operator.